The present invention relates to a drive circuit for a d.c. brushless motor, and particularly to a drive circuit for a sensorless multi-phase d.c. brushless motor that is used as a spindle motor of a hard disk drive unit (HDD) or floppy disk drive unit (FDD) for example.
Information processors such as small-scale work stations and personal computers employ a hard disk drive unit and/or floppy disk drive unit for their storage devices. These storage devices operate to write and read out data by means of a magnetic head on a track of a magnetic disk that is rotated by a spindle motor.
With the intention of providing an enhanced processing speed and ability of running large application programs for these information processors, the data recording density on the magnetic disk and the data transfer rate of the hard disk and floppy disk drive units are being improved year by year. At the same time, the size reduction and low-power performance of the hard disk and floppy disk drive units are in progress to match with the compact and low-power design of the information processors.
For the hard disk drive unit, smaller magnetic disks having diameters of 2.5 inches and 1.8 inches are being developed. The hard disk drive unit itself has been made thinner down to 10 mm to meet the demand of thinner lap-top and notebook-type personal computers.
The thickness reduction of the hard disk drive unit has promoted the thinner design of the spindle motor used in it, resulting in the elimination of the rotor position sensor (Hall element) and the development of a sensorless spindle motor of the in-hab type in which the motor windings (coils) are disposed within the rotor. In consequence, there has arisen the need of a motor drive system (sensorless drive system) for controlling accurately the rotation of a sensorless brushless motor having no rotor position sensor.
A conventional drive technique for such a sensorless brushless d.c. motor is described in the article "Sensorless drive for Brushless D.C. Motor" in Toshiba Review, Vol.45, No. 9, pp. 755-758, published by Toshiba Corp. in 1990. According to this technique, when a 3-phase sensorless brushless motor is run at the rated speed, the zero-cross points of the back electromotive force (e.m.f.) voltages Eu, Ev and Ew induced across three motor windings are detected by means of comparators, 3-phase digital pulse signals U1, V1 and W1 are produced by being timed to the detected zero-cross points, the pulse signals U1, V1 and W1 are amplified with individual power amplifiers, and the resulting drive currents are fed to the three motor windings.
A 3-phase d.c. brushless motor is driven in full-wave drive mode in which each motor winding is energized in both current directions or in half-wave drive mode in which each motor winding is energized in one current direction. At the starting of the motor when a large output torque is required, the motor is driven in full-wave drive mode, and after the motor speed enters a certain speed range where a large output torque is not required, the operation is switched to half-wave drive mode.